Advanced signaling for multi-stage transceivers

ABSTRACT

Systems and methods for advanced signaling between stages of transmitters and/or receivers in a digital communication system. One or more intermediate frequency signals and one or more control signals may share the same cable. Also, systems and methods are provided for calibrating head end receiver gain to improve subscriber unit power control loop performance.

STATEMENT OF RELATED APPLICATION

The present application is a divisional of U.S. patent application Ser.No. 09/748,760, filed Dec. 21, 2000, the contents of which areincorporated by reference herein for all purposes.

BACKGROUND OF THE INVENTION

The present invention relates to digital communication systems and moreparticularly to digital communication systems employing multi-stagetransceivers.

A point to multipoint wireless communication system represents apotentially effective solution to the problem of providing broadbandnetwork connectivity to a large number of geographically distributedpoints. Unlike optical fiber, DSL, and cable modems, there is no need toeither construct a new wired infrastructure or substantially modify awired infrastructure that has been constructed for a different purpose.The point to multipoint system includes both a hub or a head end andnumerous subscriber units associated with individual residences orbusinesses. For both the subscriber unit and head end, there is atransceiver including both a transmitter and a receiver. Both thetransmitter and receiver incorporate various stages. Special problemsarise in signaling among the stages as will be explained.

One problem arises in passing signals between physically distinctcomponents of the subscriber unit. For optimal network performance, itis desirable that the subscriber unit employ an outdoor antenna. Thesource and sink of data at a business or residence will however likelybe a computer located inside a building. If the subscriber unit were tobe integrated within one physical package inside the building inproximity to the computer, a problem arises in that RF signals willattenuated by the cable between the indoor subscriber unit housing andthe outdoor antenna. If the subscriber unit is a single physical packageoutside the house at the antenna location, another problem arises inthat high speed unmodulated data passing back and forth between thecomputer and subscriber unit may be corrupted over the relatively longdistance between the subscriber unit and computer.

Therefore, it is desirable to divide the subscriber unit componentsbetween an indoor unit and an outdoor unit. The outdoor unit includescomponents that operate at microwave frequencies. The indoor unitperforms packet processing, baseband signal processing, and processingat an intermediate frequency (IF) suitable for low-loss coupling betweenthe indoor unit and outdoor unit. The outdoor unit can then convertbetween the IF signal and the RF signal at microwave frequency.

Numerous signals, however, must pass between the indoor unit and outdoorunit including the transmitted and received IF signals, a frequencyreference signal, frequency adjustment information, and power control.Yet it is impractical to pass multiple cables between the indoor unitand outdoor unit because this would greatly complicate installation andmaintenance.

Another interstage signaling problem arises at the head end. The headend needs to regulate the upstream transmission power level used by eachsubscriber unit. If this transmission power level is too low, thesubscriber unit signal received at the head end will be too weak toaccurately recover the transmitted digital data. If this transmissionpower level is too high, the subscriber unit signal may saturate thehead end receiver or cause interference to communication links operatingoutside network 100. For example, network 100 may be a single cell of amulticellular system and excessive transmission power may causeinterference to other cells.

To regulate subscriber unit transmission power, the head end willtypically pass power control commands to subscriber units. The powercontrol commands are determined based on a received signal levelmeasurement made within the head end receiver. This measurement is madewithin the head end receiver after several stages of receiver processingincluding RF filtering, downconversion to an intermediate frequency, andvarious other filtering and processing stages. However, the signal gainthrough these premeasurement components will vary over time and amonghead end units. Because recovery of the transmitted data occurs afterthe measurement point, the variation of this signal gain and theresulting variation of subscriber unit transmission power will notaffect head end receiver power, however, an increase in subscriber unittransmission power due to receiver gain changes rather than channelconditions will potentially cause interference to cells outside network100.

It is therefore desirable to measure the head end receiver gain andcompensate for it in the operation of the subscriber unit power levelcontrol loop. However during operation it is difficult to accuratelymeasure head end receiver gain because although the signal level at themeasurement point is known, there is no easy way to measure the signallevel incident at the antenna.

What is needed are systems and methods for interstage signaling thataddress the problems noted above.

SUMMARY OF THE INVENTION

Systems and methods for advanced signaling between stages oftransmitters and/or receivers in a digital communication system areprovided by virtue of one embodiment of the present invention. Oneaspect of the invention provides for one or more intermediate frequencysignals and one or more control signals on the same cable. Anotheraspect of the invention provides systems and methods for calibratinghead end receiver gain to improve subscriber unit power control loopperformance.

Further understanding of the nature and advantages of the inventionherein may be realized by reference to the remaining portion of thespecification and the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a point to multipoint communication system according toone embodiment of the present invention.

FIG. 2 depicts a typical subscriber unit installation according to oneembodiment of the present invention.

FIG. 3 is a block diagram of the indoor portion of a subscriber unitaccording to one embodiment of the present invention.

FIG. 4 depicts the outdoor portion of a subscriber unit according to oneembodiment to the present invention.

FIG. 5 depicts how multiple signals can be combined in the frequencydomain on a single cable between indoor and outdoor portions of thesubscriber unit according to one embodiment of the present invention.

FIG. 6 depicts how head end receiver gain may be calibrated according toone embodiment of the present invention.

FIG. 7 depicts an alternative scheme for calibrating head end gainaccording to one embodiment of the present invention.

DESCRIPTION OF SPECIFIC EMBODIMENTS

A point to multipoint communication system is one example of anapplication of the present invention. The present invention is, however,of course not limited to point to multipoint communication systems butalso finds application in e.g., point to point or peer to peercommunication systems. FIG. 1 depicts a representative point tomultipoint communication system. A network 100 includes a central accesspoint or head end 102 and multiple subscriber units 104. Allcommunications typically is either to or from head end 102.Communication from head end 102 to one or more subscriber units 104 isherein referred to as downstream communication. Communication from anyone of subscriber units 104 to head end 102 is herein referred to asupstream communication. In one embodiment, different frequencies areallocated to upstream and downstream communication. In order tocoordinate communication along multiple subscriber units via a commonupstream frequency, a medium access control (MAC) protocol is provided.In one embodiment, the MAC protocol employs time division multipleaccess (TDMA) wherein time slots are allocated to individual subscriberunits for upstream transmission. An exemplary MAC protocol of this typeis the so-called DOCSIS protocol described in the Data-Over-CableService Interface Specifications, Radio Frequency InterfaceSpecifications, SP-RFI-I04-980724, (Cable Television Laboratories,1997), the contents of which are herein incorporated by reference.

Subscriber units 104 will typically be installed at residences orbusinesses. FIG. 2 depicts an exemplary installation of a subscriberunit at a building. Each subscriber unit 104 includes an interface to acomputer, set-top box, and/or computer network, telephone, television.Each subscriber unit 104 also includes circuitry for operating therelevant MAC layer protocol, circuitry for digitally processinginformation according to the relevant physical layer modulation scheme,digital to analog and analog to digital conversion circuitry, analogintermediate frequency (IF) amplification and filtering, as well asradio frequency (RF) amplification and filtering. Some of the circuitrywill be duplicated for the transmitter and receiver. If the transmitterand/or receiver take advantage of multiple antennas certain portions ofthe circuitry will also be duplicated for each antenna.

The wireless transmission medium between head end 102 and subscriberunit 104 may exploit frequencies such as microwave frequencies and/ormillimeter wave frequencies. For optimal network performance, subscriberunits 104 should employ outdoor antennas. Cable losses make itundesirable to simply transfer millimeter wave or microwave signalsbetween an indoor subscriber unit and an outdoor antenna via cable.Furthermore, it is also undesirable to locate the entire subscriber unitat an outdoor location adjacent to the antenna because of the need forhigh speed data connections to the data sinks and/or sources within theresidence.

FIG. 2 depicts the division of subscriber unit 104 into an outdoor unit202 and indoor unit 204. The outdoor unit is directly connected to anoutdoor antenna 206. The antenna may be physically integrated with theoutdoor antenna. Indoor unit 204 includes an interface to a data sinkand/or source, a MAC layer processor, baseband physical layer signalprocessing, IF amplification and filtering and conversion betweendigital and analog signals. IF transmitter and receiver signals areexchanged between indoor unit 204 and outdoor unit 202 via a cable 208.Outdoor unit 202 includes RF amplification and filtering including lownoise amplification for the receiver and power amplification for thetransmitter. Outdoor unit 202 also includes a mixer and additionalcircuitry for converting between RF and IF frequencies. Correct outputpower level and the desired transmit and receive frequencies aredetermined within indoor unit 204. According to the present invention,transmit and receive IF signals and control and synchronization signalsmay all travel via common cable 208. Cable 208 may be an RG58 cable orequivalent for residential applications. For commercial applications,LMR400 cable may be substituted.

FIG. 3 depicts indoor unit 204 according to one embodiment to thepresent invention. A subscriber unit MAC/power control block 302interfaces with a network and/or computer or other data sync or source.Block 302 translates between higher level network protocols, such as IPand the operative MAC protocol. Block 302 also requests upstreamtransmission times and coordinates upstream data transmission responsiveto a schedule grant from head end 102. Another function of block 302 isto control the upstream transmission power level of subscriber unit 104in response to downstream power control messages from head end 102.

Upstream MAC data to be transmitted begins physical layer processing bybeing transferred to a transmitter baseband processing block 304.Transmitter baseband processing block 304 performs forward errorcorrection coding, mapping of the data into symbols, other modulationrelated signal processing functions, and conversion of the digitalsignal to analog. A transmitter IF processing block 306 converts thebaseband analog signal to a signal at an intermediate frequency (IF).Transmitter IF processing block 306 also amplifies and filters theanalog signal. A band pass filter 308 further filters the IF analogsignal. The IF signal output of the band pass filter 308 is coupled tocable 208. As will be explained, many signals share cable 208 by beingfrequency multiplexed.

Also sharing cable 208 is a receiver IF signal from outdoor unit 202.The signal passes through a band pass filter 310 and then onto receiverIF processing block 312. Receiver IF processing block 312 filters andamplifies the IF signal and converts it to a base band analog signal. Areceiver baseband processing block 314 converts the analog base bandsignal to digital, filters the digital signal, performs variousdemodulation related processing functions, performs error correctiondecoding, and presents the recovered data to block 302.

The actual transmit and receive RF frequencies are set by the operationof outdoor unit 202 under the control of block 302 within indoor unit204. Data representing the desired frequencies is sent from indoor unit204 to outdoor unit 202. Also indoor unit 204 sets the transmissionpower level of outdoor unit 202. Various other status information suchas synthesizer lock state, power fail state, etc. may be exchangedbetween outdoor unit 202 and indoor unit 204. Accordingly, indoor unit204 includes a programmable interface controller or a PIC controller316. In one embodiment, PIC controller 316 serves as a data interfacebetween indoor unit 204 and outdoor unit 202. PIC controller 316 is aPIC 16F877 controller available from Microchip Technology, Inc. ofChandler, Ariz. PIC controller 316 also performs control and monitoringfunctions for transmitter IF processing block 306 and receiver IFprocessing block 312. These IF control functions are not discussed here.

Power and frequency control commands from block 302 to outdoor unit 202are processed by PIC controller 316. The data to be transferred tooutdoor unit 202 is Manchester encoded by a Manchester encoder/decoder318. The Manchester encoded data then passes through a band pass filter320 which is coupled to cable 208. In one embodiment, PIC controller 316and Manchester encoder/decoder 318 operate a half duplex 125 KbpsManchester encoded two way serial communication channel.

To transmit power control information, first, an eight bit data wordrepresenting a desired power attenuation level is transmitted to outdoorunit 202 via PIC controller 316. However the power adjustment itselfshould occur at a precise time controlled by block 302. Accordingly, ablanking signal generator 322 outputs a 48 MHz signal that is keyed onand off for power adjustment timing control. When the 48 MHz signal isturned on, that indicates that a previously loaded power adjustment fromPIC control 316 should be put into effect immediately. This 48 MHzcarrier is coupled to cable 208 via a band pass filter 324.

Indoor unit 204 and outdoor unit 202 should also share a commonunderstanding of system timing. A timing source 326 generates a 24 MHzreference signal. The synchronization signal flows to block 302 andother elements of indoor unit 204 and is coupled to cable 208 via bandpass filter 328.

A 48 volt DC power supply voltage is also passed to outdoor unit 202 viacable 208. This voltage is generated by a power supply 330 and filteredby a low pass filter 332.

FIG. 4 depicts outdoor unit 202 according to one embodiment of thepresent invention. For the transmit signal, a band pass filter 402isolates the transmit IF signal and passes it along to a “conversion toRF” block 404. Block 404 incorporates a mixer to convert the IF signalto RF frequency by mixing the IF signal with a local oscillator signalderived from the 24 MHz reference signal. The RF signal is thenamplified by a power amplifier 406 and passed through a diplexer 408which is coupled to antenna 206.

The RF receive signal from head end 102 is received by antenna 206 andis input to diplexer 408 and from there passed along to an analogreceiver block 410. Analog receiver block 410 performs low noiseamplification, RF filtering, and conversion to the receiver IFfrequency. Block 410 incorporates a mixer which also takes advantage ofthe 24 MHz reference signal in converting to the IF frequency. Thereceiver IF signal is filtered by a band pass filter 412 prior to beingforwarded to indoor unit 204 via cable 208.

A low pass filter 414 isolates the DC power, 24 MHz reference signal, 48MHz blanking signal, and the Manchester encoded data stream from thetransmitter IF and receiver IF signals. Low pass filter 414 preferablyhas a 100 MHz cutoff. The 48 MHz blanking signal and the 24 MHzreference signal are obtained from the output of low pass filter 414.

The output of low pass filter 414 is also coupled to the input ofanother low pass filter 416 that preferably has a 10 MHz cut off. TheManchester serial data is coupled from low pass filter 416 via a bandpass filter 418 to a Manchester encoder/decoder 420. Band pass filter418 preferably has a 750 KHz center frequency and a 500 KHz bandwidth.Manchester encoder/decoder 420 decodes data received from indoor unit204 and encodes data transmitted to indoor unit 204. A PIC controller422 similar to PIC controller 316 controls transmission frequency,reception frequency, and output power according to the data receivedfrom indoor unit 204 via Manchester encoder/decoder 420. PIC controller422 is similar to PIC controller 316 in indoor unit 204. Transmissionand reception frequencies are controlled by varying local oscillatorfrequencies within RF conversion block 404 and analog RF receiver block410. A new power level is latched into power amplifier 406 by a latch424 when the 48 MHz blanking signal is activated.

The output of low pass filter 416 is also coupled to the input of afurther low pass filter 426 that preferably has a cutoff frequency of 1MHz. DC power is obtained from the output of low pass filter 426. A DCto DC converter 428 generates needed supply voltages based on the 48volt DC power obtained from indoor unit 204.

FIG. 5 depicts the signals on cable 208 in the frequency domain. Themodulated receiver IF signal is at 426 MHz. The modulated transmitter IFsignal is at 330 MHz. The blanking signal which is used to latch newdesired power level is a sinusoid at 48 MHz. The blanking signal iseither on or off. When the signal is turned on a new power level isadopted by changing the attenuation within power amplifier 406. Thesignal is typically turned on right before a transmission by subscriberunit 104. The blanking signal is only one example of an amplitude shiftkey (ASK) waveform that can signal a switching time by changing itsamplitude.

The 24 MHz sinusoid is the frequency reference signal. The Manchesterencoded control data which travels between indoor unit 204 and outdoorunit 202 is at 750 KHz with only one end transmitting at a time. Thereis also the 48 volt DC power signal. At both ends of cable 208 thesevarious signals are coupled to the cable by bandpass and lowpass filtersas shown in FIG. 3 and FIG. 4. It will be appreciated also thatimpedances must be matched appropriately for all of the filters coupledto cable 208.

Another aspect of the present invention addresses interstage signalingto measure and adjust the gain of the receiver of head end 102. Foroptimal network performance head end 102 operates a power control loopthat measures the received signal strength of signals transmittedupstream by individual subscriber units 104 and sends downstream signalsto adjust their power for optimal network operation. To support accurateoperation of this power control loop, head end 102, according to oneembodiment of the present invention, can measure the gain from theantenna to the point where receiver signal strength is measured for thepower control loop.

FIG. 6 depicts elements of head end 102 according to one embodiment ofthe present invention. Like with subscriber unit 104, functions may bedivided between an indoor unit and an outdoor unit but this division isnot shown here because it is not important to understanding the head endreceiver gain calibration function. A MAC layer processor 602 controlsoverall MAC layer operation and provides MAC layer framing and deframingservices. Data to be transmitted downstream is transferred in the formof MAC layer frames to a head end transmitter system 604. The details ofthe operation of head end transmitter system 604 are not important tothe present invention and therefore not discussed in detail here. Theoutput of head end transmitter system 604 is the RF signal which isforwarded to a diplexer 606 which is in turn coupled to an antenna 607.

Upstream signals are incident on antenna 607 and coupled throughdiplexer 606 to head end receiver analog chain 608. Head end receiveranalog chain 608 includes amplification and filtering at the RFfrequency and downconversion to an IF frequency, further filtering andamplification at the IF frequency, as well as downconversion to ananalog base band signal. An analog to digital converter 610 converts theanalog baseband signal to a digital signal. Further digital processingat baseband is provided by head end digital baseband receiver system 612which performs further filtering, demodulation, and error correctiondecoding. The output of block 612 is MAC layer data that is forwarded toMAC layer processor 602. MAC layer processor 602 operates as theinterface to a backbone network employing, e.g., IP.

To implement the power control loop, signal strength is measured at theinput to analog to digital converter 610. MAC layer processor 602 takesthis signal measurement and adjusts subscriber unit transmission powerto maintain the signal strength within a desired range. In order to dothis, MAC layer processor 602 sends special power control messagesdownstream through head end transmitter system 604. However, the signalstrength measured at the input to analog to digital converter 610 willdepend on the gain through head end receiver analog chain 608.

As the gain varies due to, for example, temperature variation, MAC layerprocessor 602 will adjust subscriber unit output power for this gainvariation rather than any change in the subscriber unit's own poweroutput or changes in attenuation through the wireless transmissionmedium. It is therefore desirable to measure the gain through head endreceiver analog chain 608 so that subscriber unit output power will beset correctly. MAC layer processor 602 preferably chooses a time slotwhen no subscriber unit is transmitting upstream through head endreceiver 608. This time slot is then used for gain measurement.

When MAC layer processor 602 detects that such a “quiet slot” is coming,it activates a calibration control block 614. Calibration control block614 then measures the signal level at the input to analog to digitalconverter 610. The ratio of the measured signal level over the knownthermal noise level at antenna 607 will then be the gain throughdiplexer 606 and head end receiver analog chain 608. This gain is thenadjusted by calibration control block 614 so that the power control loopwill continue to set the subscriber unit output power within a rangethat ensures accurate reception and that does not unduly interfere withupstream transmissions to other head ends. This gain adjustments isperformed by changing an attenuator setting within head end analog chain608. Preferably this calibration operation will only occupy a fewmilliseconds and will be performed once or twice a day. The necessaryfrequency of calibrations may depend on the degree of temperaturevariation at the head end location.

It should be noted that in one embodiment the signal that goes to headend receiver analog chain 608 and analog to digital converter 610includes multiple frequency multiplexed upstream signals from differentsubscriber units that are then separated within head end digital baseband receiver system 612. Calibration control will then require that allof these upstream subchannels be quiet during the calibration period.

FIG. 7 depicts an alternative embodiment of head end 102 that employs asingle transmit antenna but two receiver antennas. There are now twoantennas 607. One of the antennas 607 is coupled to diplexer 606 and isthus used for both transmitter and receiver operation. There is a singlehead end transmitter 604 and MAC layer processor 602 as in FIG. 6. Alsothere is a single calibration control block 614. There is also a secondantenna 607 used only for receiver operation to improve receiverperformance by exploiting spatial diversity. There are head end analogchains 608 for each receiver antenna. There are also two analog todigital converters 610. A head end digital base band receiver system612′ performs all the same functions as head end digital base bandreceiver system 612 but also incorporates signal processing tobeneficially combine the digital signals received from both antennas.Power control operation is now based on a combined measurement of theinputs of the two analog to digital converters 610.

The gains of both head end chains 608 may be calibrated independently aswas described in reference to FIG. 6, that is, by waiting for a quiettime in upstream transmissions and then taking the ratio of measuredsignal level to thermal noise level to be the gain through each chainwhere one of the head end analog chains 608 will also have a gainassociated with diplexer 606. The gains are measured independently andadjusted independently.

Alternatively, in this dual antenna embodiment, MAC layer processor 602need not wait for a quiet time but may instead monitor receiverperformance by exploiting an error rate indication developed by head enddigital base band receiver system 612′. When the error rate is low foran extended time period, one head end analog chain 608 may bedeactivated while receiver operation continues to rely on the other one.Because of the low error rate, channel quality is deemed to besufficient to permit foregoing the advantages of spatial diversityduring the calibration period. Now the gain for the inactive head endanalog chain 608 may be calibrated while upstream communicationcontinues uninterrupted. The head end analog chains 608 can thenexchange roles with the newly calibrated analog chain being reactivatedto allow for calibration of the one that had been left on. In oneembodiment, deactivation of a head end analog chain involves switchingthe input to the chain into a dummy load and disconnecting it from theantenna 607.

It is understood that the examples and the embodiments described hereinare for illustrative purposes only and that various modifications orchanges in light thereof will be suggested to persons skilled in the artand are to be included within the full scope and purview of thisapplication and scope of the appended claims and their full scope ofequivalents. All publications, patents, and patent applications citedherein are hereby incorporated by reference in their entirety.

1. A method for outputting both a high frequency data modulated waveformand an RF power output control signal onto a common transmission medium,said method comprising: transmitting said high frequency data modulatedwaveform at a first frequency onto said common transmission medium;transmitting a first control signal indicating a desired RF power outputlevel onto said common transmission medium; said first control signalbeing transmitted at a second frequency different than said firstfrequency; and transmitting a second control signal indicating a desiredRF power output level switching time, said second control signal beingtransmitted at a third frequency different from said second frequencyand said first frequency.
 2. The method of claim 1 wherein said secondcontrol signal comprises an amplitude shift key modulated carrier wave.3. The method of claim 1 further comprising outputting a DC power signalonto said common transmission medium.
 4. The method of claim 1 furthercomprising transmitting a third control signal comprising a timingreference waveform onto said common transmission medium.
 5. A method foraccepting both a high frequency data modulated waveform and an RF poweroutput control signal from a common transmission medium, said methodcomprising: receiving said high frequency data modulated waveform at afirst frequency from said common transmission medium; receiving a firstcontrol signal indicating a desired RF power output level from saidcommon transmission medium; said first control signal being received ata second frequency different than said first frequency; and receiving asecond control signal indicating a desired RF power output levelswitching time, said second control signal being received at a thirdfrequency different from said second frequency and said first frequency.6. The method of claim 5 wherein said second control signal comprises anamplitude shift key modulated carrier wave.
 7. The method of claim 5further comprising receiving a DC power signal from said commontransmission medium.
 8. The method of claim 5 further comprisingreceiving a third control signal comprising a timing reference waveformfrom said common transmission medium.
 9. A system for outputting both ahigh frequency data modulated waveform and an RF power output controlsignal onto a common transmission medium, said system comprising: atransmitter system that transmits said high frequency data modulatedwaveform at a first frequency onto said common transmission medium; andan interface control signal generator system that generates a firstcontrol signal indicating a desired RF power output level onto saidcommon transmission medium; said first control signal being transmittedat a second frequency different than said first frequency, and thatgenerates a second control signal indicating a desired RF power outputlevel switching time, said second control signal being transmitted at athird frequency different from said second frequency and said firstfrequency.
 10. The system of claim 9 wherein said second control signalcomprises an amplitude shift key modulated carrier wave.
 11. The systemof claim 9 further comprising a power supply that outputs a DC powersignal onto said common transmission medium.
 12. The system of claim 9further comprising a timing source that outputs a timing referencewaveform onto said common transmission medium.
 13. A system foraccepting both a high frequency data modulated waveform and an RF poweroutput control signal from a common transmission medium, said systemcomprising: a transmitter system that obtains said high frequency datamodulated waveform at a first frequency from said common transmissionmedium; an interface control system that receives a first control signalindicating a desired RF power output level from said common transmissionmedium; said first control signal being received at a second frequencydifferent than said first frequency; and a power level control systemthat receives a second control signal indicating a desired RF poweroutput level switching time, said second control signal being receivedat a third frequency different from said second frequency and said firstfrequency.
 14. The system of claim 13 wherein said second control signalcomprises an amplitude shift key modulated carrier wave.
 15. The systemof claim 13 further comprising a power supply that receives a DC powersignal from said common transmission medium.
 16. The system of claim 13further comprising a receiver system that obtains a third control signalcomprising a timing reference waveform from said common transmissionmedium, said third control signal being received at a third frequencydifferent from said first frequency and said second frequency. 17.Apparatus for outputting both a high frequency data modulated waveformand an RF power output control signal onto a common transmission medium,said apparatus comprising: means for transmitting said high frequencydata modulated waveform at a first frequency onto said commontransmission medium; means for transmitting a first control signalindicating a desired RF power output level onto said common transmissionmedium; said first control signal being transmitted at a secondfrequency different than said first frequency; and means fortransmitting a second control signal indicating a desired RF poweroutput level switching time, said second control signal beingtransmitted at a third frequency different from said second frequencyand said first frequency.
 18. Apparatus for accepting both a highfrequency data modulated waveform and an RF power output control signalfrom a common transmission medium, said apparatus comprising: means forreceiving said high frequency data modulated waveform at a firstfrequency from said common transmission medium; means for receiving afirst control signal indicating a desired RF power output level fromsaid common transmission medium; said first control signal beingreceived at a second frequency different than said first frequency; andmeans for receiving a second control signal indicating a desired RFpower output level switching time, said second control signal beingreceived at a third frequency different from said second frequency andsaid first frequency.